I. Field of the Invention
The present invention relates generally to a battery system for use by a vehicle utilizing a nickel hydroxide based battery (e.g. nickel-hydrogen, nickel metal-hydride, nickel-iron, nickel-zinc, nickel-cadmium), and more particularly, to such a system in which, for example, the nickel hydroxide battery is recharged at a temperature which is substantially lower than the temperature at which discharge is performed.
II. Description of the Prior Art
It is well recognized that optimum performance for nickel hydrogen spacecraft batteries requires that their temperature be controlled in the range of +20.degree. C. to -10.degree. C. Higher temperatures lead to lowered capacity due to the early on-set of the parasitic electrolysis reaction, e.g., EQU H.sub.2 O+2e.sup.- =OH.sup.- +1/2O.sub.2 ( 1)
Reaction (1) has been recognized in the prior art to reduce capacity as the operating temperature of the battery is increased. A key to the present invention has been the recognition that the critical temperature is that at which the battery is recharged and reaction (1) competes with the normal recharge reaction, e.g., EQU Ni(OH).sub.2 +OH.sup.- =NiOOH+H.sub.2 O+e.sup.- ( 2)
Lower temperatures on the other hand have been found to lead to batteries which fail to operate. The reasons for this failure, prior to the present invention, were unclear as the freezing point of the electrolyte (-61.degree. C.) is well below the minimum operation temperature. The second key to the present invention has been the recognition that the lower temperature limit is due to a limitation in the ability of the battery to sustain high rate discharge required in satellites situated in a geosynchronous orbit (e.g., .about.C/1.5) as opposed to the lower rate recharge (e.g., .about.C/10).
Typical of the prior art as it relates to charging and discharging nickel batteries is U.S. Pat. No. 4,680,241 to Dyer. The Dyer patent concerns a method for partially or fully restoring the lost capacities of nickel batteries. In this instance, a nickel battery is cycled at least 10 times, with each cycle including a discharging step during which the capacity achieved at the end of the previous cycle is reduced by at least 5 percent, and a charging step. The charging rate employed during the charging step is greater than about C/10 per hour. Moreover, while the ratio of the amount of charge delivered to the battery during the charging step of each cycle to the amount of charge withdrawn from the battery during the previous cycle is greater than one, this ratio is chosen so that the temperature of the electrolyte of the battery does not exceed about 30.degree. C.
It is clear, however, that the Dyer patent does not address the particular problem which the present invention is intended to solve.
Also known are rechargeable batteries with various forms of temperature control. For example, U.S. Pat. No. 5,229,702 to Boehling et al. describes protection of a rechargeable battery in a power system by placing it in an insulated chamber under temperature control of thermoelectric devices operated by the power system to pump heat out of the chamber or by the battery to pump heat into the chamber. U.S. Pat. No. 5,141,826 to Bohm et al. describes a high-energy battery with a plurality of individual cells in a housing through which a coolant flows, the coolant being guided such that it thermally affects only one or both end faces of the cells. U.S. Pat. No. 4,324,845 to Stockel describes a heat pipe which, using a working fluid such as Freon 21, connects a rechargeable cell to a radiator.
Various constructions of cooling apparatus are also known in the literature. For example, U.S. Pat. No. 5,071,652 to Jones et al. describe a plurality of adjacent cell modules separated by heat transfer members in the form of fins. U.S. Pat. No. 5,015,545 to Brooks describes a battery housing with air gaps to assure substantially uniform cooling of all cells. U.S. Pat. No. 4,865,929 to Eck describes a distributor panel with suitable openings to guide flow of coolant around battery cells to achieve optimum cooling.
As described in copending and commonly assigned U.S. application Ser. No. 08/182,224, filed Jan. 14, 1994, entitled "Satellite Battery Thermal/Capacity Design", the disclosure of which is hereby incorporated in its entirety, by reference, the capacity of a nickel hydroxide based battery system is increased when its recharge is carried out at a much lower temperature than its discharge. For satellite applications this is reduced to practice by suitably sizing the space radiator area and resetting the lower heater set point between charge and eclipse discharge.
The intrinsic capability of a space battery to be cold charged does not exist for other battery applications, however. Electric vehicles represent an application where cold charge could be of particular utility as gravimetric energy density and low cost are both critical. However, thermal management of these batteries has usually focused on the need to cool the battery during discharge while being driven. Such cooling typically exchanges heat between the vehicle and moving air. Thus the temperature of the battery based on this heat exchange approach can be no lower than the surrounding air. As vehicle recharge is envisioned as taking place inside the owner's garage, there is no equivalent heat sink to the spacecraft battery for carrying out cold charge.
It was in light of the state of the technology as just described that the present invention has been conceived and is now reduced to practice.